Hi,
as said before the IAP cluster must be in the same subent (Vlan) , since it use a layer 2 IAP use layer 2 protocol to exchange management traffic.
please try to check your switch configuration and see if all your IAP are in the same VLAN or not , and be sure that there is no firewall or routeur that can block the traffic.
to be honnest in your case a Controllre based architecture is recommanded , you can also use an Airwave Appliance , that will give you more management and visiblity option.
Regards